Hahahha lol, the flush is just the last week of your flowering period, something between 7 and 14 days before you are harvesting, for example your plant is more indica with a flowering period of around 8 weeks, this means that at week 7 you stop giving nutrients and only water with clean water, i do it normally with a bit more water than i use to give them just to be sure they drain all the remaining nutrients in the soil to have a better smoke, better taste, enjoy your grow, and if you think that my answer helped you please tag it as solution 😁 happy growing 🌱